[sdiy] Prophet 5 rev2 conundrum

brianw brianw at audiobanshee.com
Wed May 3 01:49:10 CEST 2023

If you have a fancy oscilloscope with analog input channels and digital storage, then you might be able to capture the signals during the tuning process, and from that information determine what's going wrong. You'd probably have to first look at a working board to understand what's "normal," and only then might you be able to make sense of the "bad" board. You'd probably need to trigger on one or more of the "digital" signals controlling the multiplexers, but also record the analog mux voltage during the process.

It seems that you've confirmed that each individual IC is good, but perhaps the voltages are in an unexpected range during the tuning.

I'm looking at this problem from the point of view of a hardware / firmware developer who might be trying to design or debug the tuning algorithm. The algorithm is probably not too complicated, but you can't get visibility of the problem simply by probing steady-state signals or replacing socketed parts. You probably have to see the process in motion - thus the digital recording oscilloscope - before you can get to the bottom of a problem like this. I recall that the Technical Manual describes the tuning process in some detail, so familiarize yourself with that, if you haven't already.

Granted, it shouldn't be this hard to repair a synth, but these old Rev 2 Prophets are rare enough that it's surely worth the extra effort.

If you want to share images (scope shots) privately, then I'd be willing to offer some time looking at the information. I'm a P5R2 owner, too, but I do not own one of these fancy oscilloscopes - only when consulting with clients on hardware and firmware designs have I enjoyed the access to such tools. They can help solve a lot of complicated problems. Hopefully you can rent one if you don't already own one.

Brian Willoughby

p.s. I got addicted to a particular Tektronix model while working with Mackie on several products. When my contract was over there, I looked into buying the same model for my personal lab. Turns out that this particular 'scope is $16,000. Needless to say, I didn't order one from Tektronix. They might still be viable to rent. Places like Seattle have a vibrant used market for 'scopes, and I'm sure some folks would be willing to rent one out. Hopefully you're not based in the middle of nowhere without access to a community.

On May 2, 2023, at 9:41 AM, g m montalbano via Synth-diy <synth-diy at synth-diy.org> wrote:
> I've been working on these for twenty years, and thought I'd seen pretty much everything that could go wrong with them.
> Now, I have one that works perfectly except for the fact that it will not go through the tune procedure, either at powerup or when the Tune button is pressed.
> The confusing part is that the fault has been narrowed down to the voice board: if I swap in a working voice board, the unit functions correctly; if I install this voice board in another unit, the problem follows.
> As far as I can see, the ONLY parts on the voice board that relate to tuning are the two 4051 multiplexers (U423 and U428) and the LM311 comparator (and its associated resistors & caps). All of these components have been replaced; traces and resistors have been checked &  measured. All traces to the 60 pin connector between the CPU and Voice boards are good, as is the 60 pin cable.
> EVERYTHING is there, and everything works correctly except for Tune.
> I've been hitting my head against this for a couple of weeks, and would appreciate any suggestions as to where to look.
> Thanks.
> ~GMM

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